Water at a rate of 0.32 L/s and a temperature of 27°C enters a No. 18 BWG 1.6 cm condenser tube made of nickel chromium steel ( k = 26 W/(m K) ) . The tube is 3 m long and its outside is heated by steam condensing at 50°C. Under these conditions, the average heat transfer coefficient on the water side is 10 kW/(m2 K), and the heat transfer coefficient on the steam side may be taken as 11.3 kW/(m2 K). On the interior of the tube, however, there is a scale having a thermal conductance equivalent to 5.6 kW/(m2 K). (a) Calculate the overall heat transfer coefficient U per square meter of exterior surface area. (b) Calculate the exit temperature of the water.
GIVEN
• A nickel chromium steel condenser tube with water inside and condensing steam outside
• Water flow rate v
= 0.32 L/s= 3.2*10-4 m3/s
• Water inlet temperature (Tw,in) = 27°C
• Tube: No. 18 BWG 1.6 cm
• Steel thermal conductivity (kst) = 26 W/(m K)
• Tube length (L) = 3 m
• Steam temperature (Ts) = 50°C
• Average water- side transfer coefficient ( )ih = 10 kW/(m2 K)
• Average steam transfer coefficient ( )oh = 11.3 kW/(m2 K)
• Interior scaling conductance (1/Ri) = 5.6 kW/(m2 K)
FIND
(a) The overall heat transfer coefficient (U) based on exterior surface area
(b) The outlet water temperature (Tw,in)
SKETCH
PROPERTIES AND CONSTANT
for No. 18 BWG 1.6 cm tube
Inside Diameter (Di) = 1.34 cm
Outside Diameter (Do) = 1.6 cm
the density of water at 30°C (?) = 995.7 kg/m3;
the specific heat (cpw) = 4176 W/(kg K)
(a) The overall heat transfer coefficient can be calculated
where Ro = exterior scaling resistance = 0
(b) The number of transfer units is
Since the heat capacity rate of the steam is essentially infinite, Cmin/Cmax = 0.
(both are the same for Cmin/Cmax = 0): e ? 0.15
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